[Python-ideas] Support WHATWG versions of legacy encodings

Rob Speer rspeer at luminoso.com
Tue Jan 9 18:56:01 EST 2018

Oh that's interesting. So it seems to be Python that's the exception here.

Would we really be able to add entries to character mappings that haven't
changed since Python 2.0?

On Tue, 9 Jan 2018 at 16:53 Ivan Pozdeev via Python-ideas <
python-ideas at python.org> wrote:

> First of all, many thanks for such a excellently writen letter. It was a
> real pleasure to read.
> On 10.01.2018 0:15, Rob Speer wrote:
> Hi! I joined this list because I'm interested in filling a gap in Python's
> standard library, relating to text encodings.
> There is an encoding with no name of its own. It's supported by every
> current web browser and standardized by WHATWG. It's so prevalent that if
> you ask a Web browser to decode "iso-8859-1" or "windows-1252", you will
> get this encoding _instead_. It is probably the second or third most common
> text encoding in the world. And Python doesn't quite support it.
> You can see the character table for this encoding at:
> https://encoding.spec.whatwg.org/index-windows-1252.txt
> For the sake of discussion, let's call this encoding "web-1252". WHATWG
> calls it "windows-1252", but notice that it's subtly different from
> Python's "windows-1252" encoding. Python's windows-1252 has bytes that are
> undefined:
> >>> b'\x90'.decode('windows-1252')
> UnicodeDecodeError: 'charmap' codec can't decode byte 0x90 in position 0:
> character maps to <undefined>
> In web-1252, the bytes that are undefined according to windows-1252 map to
> the control characters in those positions in iso-8859-1 -- that is, the
> Unicode codepoints with the same number as the byte. In web-1252, b'\x90'
> would decode as '\u0090'.
> According to https://en.wikipedia.org/wiki/Windows-1252 , Windows does
> the same:
>     "According to the information on Microsoft's and the Unicode
> Consortium's websites, positions 81, 8D, 8F, 90, and 9D are unused;
> however, the Windows API MultiByteToWideChar
> <http://msdn.microsoft.com/en-us/library/windows/desktop/dd319072%28v=vs.85%29.aspx>
> maps these to the corresponding C1 control codes
> <https://en.wikipedia.org/wiki/C0_and_C1_control_codes>."
> And in ISO-8859-1, the same handling is done for unused code points even
> by the standard ( https://en.wikipedia.org/wiki/ISO/IEC_8859-1 ) :
>     "*ISO-8859-1* is the IANA
> <https://en.wikipedia.org/wiki/Internet_Assigned_Numbers_Authority>
> preferred name for this standard when supplemented with the C0 and C1
> control codes <https://en.wikipedia.org/wiki/C0_and_C1_control_codes>
> from ISO/IEC 6429 <https://en.wikipedia.org/wiki/ISO/IEC_6429>"
> And what would you think -- these "C1 control codes" are also the
> corresponding Unicode points! (
> https://en.wikipedia.org/wiki/Latin-1_Supplement_(Unicode_block) )
> Since Windows is pretty much the reference implementation for
> "windows-xxxx" encodings, it even makes sense to alter the existing
> encodings rather than add new ones.
> This may seem like a silly encoding that encourages doing horrible things
> with text. That's pretty much the case. But there's a reason every Web
> browser implements it:
> - It's compatible with windows-1252
> - Any sequence of bytes can be round-tripped through it without losing
> information
> It's not just this one encoding. WHATWG's encoding standard (
> https://encoding.spec.whatwg.org/) contains modified versions of
> windows-1250 through windows-1258 and windows-874.
> Support for these encodings matters to me, in part, because I maintain a
> Unicode data-cleaning library, "ftfy". One thing it does is to detect and
> undo encoding/decoding errors that cause mojibake, as long as they're
> detectible and reversible. Looking at real-world examples of text that has
> been damaged by mojibake, it's clear that lots of text is transferred
> through what I'm calling the "web-1252" encoding, in a way that's
> incompatible with Python's "windows-1252".
> In order to be able to work with and fix this kind of text, ftfy registers
> new codecs -- and I implemented this even before I knew that they were
> standardized in Web browsers. When ftfy is imported, you can decode text as
> "sloppy-windows-1252" (the name I chose for this encoding), for example.
> ftfy can tell people a sequence of steps that they can use in the future
> to fix text that's like the text they provided. Very often, these steps
> require the sloppy-windows-1252 or sloppy-windows-1251 encoding, which
> means the steps only work with ftfy imported, even for people who are not
> using the features of ftfy.
> Support for these encodings also seems highly relevant to people who use
> Python for web scraping, as it would be desirable to maximize compatibility
> with what a Web browser would do.
> This really seems like it belongs in the standard library instead of being
> an incidental feature of my library. I know that code in the standard
> library has "one foot in the grave". I _want_ these legacy encodings to
> have one foot in the grave. But some of them are extremely common, and
> Python code should be able to deal with them.
> Adding these encodings to Python would be straightforward to implement.
> Does this require a PEP, a pull request, or further discussion?
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> --
> Regards,
> Ivan
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